1. Field of the Invention
The invention relates in general to a transflective liquid crystal display (LCD), and more particularly to a transflective LCD having a transmissive pixel and a reflective pixel, which are respectively controlled by different pixel voltages.
2. Description of the Related Art
In a conventional transflective liquid crystal display (LCD), such as a vertically-aligned (VA) mode transflective LCD, each pixel has a pixel electrode and a reflective electrode electrically connected to each other. The pixel electrode and the reflective electrode respectively form a transmissive region and a reflective region, and the pixel electrode and the reflective electrode are driven by the same pixel voltage. As for the transmissive region, the pixel voltage influences the transmittance rate of backlight passing through a liquid crystal layer in the transmissive region. As for the reflective region, the pixel voltage influences the transmittance rate of environmental light passing through the liquid crystal layer in the reflective region, it is emitted from an environment light source, incident to the liquid crystal layer and then reflected outward by a reflective layer.
The relationship between the pixel voltage and the transmittance rate in the transmissive region, and the relationship between the pixel voltage and the transmittance rate in the reflective region respectively form different voltage-transmission (V-T) curves. Usually, both of the maximum transmittance rates of the V-T curve of the transmissive/reflective region are defined as 100%. FIG. 1 depicts V-T curves of a transmissive region and a reflective region in a conventional transflective LCD.
As shown in FIG. 1, curve 101 is the V-T curve in the transmissive region and curve 102 is the V-T curve in the reflective region. When the pixel voltage is higher than a threshold voltage, the transmissive region and the reflective region start to respectively generate transmittance rate variations. At the same pixel voltage V, the transmittance rate Lt and the transmittance rate Lr, respectively corresponding to the transmissive region and the reflective region are not the same, such that the gray levels of the transmissive region and the reflective region are different from each other and the trends of the two curves are also different from each other. Therefore, in the pixel structure of the conventional transflective LCD, it is difficult for one pixel voltage V to make the transmissive region and the reflective region display the same gray level. Therefore, the transmissive region and the reflective region in one pixel cannot simultaneously reach the desired displaying effect.
For example, one image has to be displayed by four pixels A, B, C and D having different gray levels. However, the same set of pixel voltages are used, so it is impossible to make the transmissive region and the reflective region simultaneously display the four same gray levels. The above-mentioned problem may be solved in the conventional transflective LCD by designing the transmissive region and the reflective region to have different cell gaps (also referred to as a “dual cell gaps”), for example, the cell gap of the reflective region is about one half that of the transmissive region. However, the manufacturing processes are complicated and expensive.
Operationally, the light source of the reflective region comes from the outside environment. When the environmental light source changes, for example, when the light source changes from the outdoor sunlight to the indoor daylight lamp, or when the sunlight changes with time, the brightness and the color presented by the reflective region also change. The color is the result obtained after the light passes through the color filter. However, the light of the transmissive region only comes from the backlight source, and its brightness and color cannot change with the change of the environmental light. Therefore, if the transmissive region and the reflective region are driven by the same pixel voltage, the brightness or colors co-presented by the transmissive region and the reflective region often deviate from the optimum settings, which are set when the product is finished and ready to be shipped out, so that the incongruent condition is obtained when the environmental light changes with the changes of the time and the environment.
Therefore, it is an important subject in the industry to make the displayed gray level generated by the transmissive light the same as the displayed gray level generated by the reflective light in an LCD. It is further important that the brightness and the color representation of the reflective region are not changed with the variation of the environmental light so that the image displaying quality can be enhanced.